1. Technical Field
This invention relates to an apparatus and method for measuring spectral fundus image data. In particular, it relates to an apparatus and method for measuring spectral fundus image data that make spectral characteristics uniform within a specified wavelength range. It also relates to an apparatus and method for measuring spectral fundus images data that coordinate a large number of image data taken at different time points with high accuracy over a specified wavelength range.
2. Related Art
Fundus observation is doubtless important in ophthalmic diagnosis. At present, anomaly findings are obtained by diagnosing the eye fundus by means of colored fundus images, fluorescent contrast images, etc. from a fundus camera. If it is possible to measure quantitatively oxygen saturation degree on the fundus and constituent substances distributed in the retina, there is a possibility of finding out the functions of fine parts of the retina, which is considered to be greatly useful in clinical applications. Further, if spectral distribution of substances in the retina is clarified by spectral analyses, there is a possibility of analyzing the substances in the retina from the spectral images.
However, most of the studies carried out up to now are far from in full-scale. Full-scale image measurement is considered to meet such conditions as: (a) being capable of obtaining high quality images, and (b) being capable of measuring spectral images with a higher degree of wavelength analysis over a wide wavelength band. Such an image measurement method is occasionally called hyper-spectral imaging. Advent of the liquid crystal wavelength tunable filter has made it possible to obtain spectral images relatively easily. Using a number of spectral images of different wavelengths makes it possible to examine spectral characteristics of substances in detail and to extract constituents having various known spectral distributions.
While full-scale measurement of the living organism is the subject for the future, preliminary studies have been carried out at two to four wavelengths or so on animals as models, in which measurements are used to confirm if this technique can cope with measuring the human eye. Besides, an examination is being carried out using an apparatus, that can be applied to the human eye, for obtaining spectral images of the fundus by separating light using a diffraction grating and scanning the fundus.
While the hyper-spectral imaging is a technique in the spotlight and is used to obtain spectral images of the fundus, it is hard to perform accurate analyses because the amount of light of spectral images obtained varies greatly by the wavelength. Moreover, the hyper-spectral light separation with a light amount without putting burden on humans has yet to be realized, and an apparatus enabling it has yet to be realized either.
As typical light separation devices, such ones may be enumerated as: the diffraction gratings, prisms, etalons, and filters. Up to now, diffraction gratings and prisms have been often used in spectral measurement of tunable wavelength. Recently, however, the advent of the liquid crystal wavelength tunable filter has made it possible to take spectral images at any wavelength and measure spectral images easily. Since the liquid crystal wavelength tunable filter is fundamentally a parallel flat plate, it can be easily installed in the optical system, and its optical performance can be easily maintained. Therefore, it is often used in obtaining spectral images using a microscope. It is also used in studies in which spectral images obtained are synthesized to create natural images.
However, there has been a problem of insufficient amount of light on the short wavelength side and excessive amount of light on the long wavelength side (in the range from 500 nm to 700 nm) due to spectral characteristics of the liquid crystal wavelength tunable filter, the light source (halogen lamp, etc.), and the CCD.
Besides, as restricted by for example the wavelength tunable time of the liquid crystal wavelength tunable filter and the exposure time of the camera, it takes about 20 seconds to take images at every 10 nm in the wavelength range from 510 nm to 720 nm. Because alignment between the eye and the apparatus varies during that time, there has been another problem that the spectral images taken of the same part are displaced from each other.
An object of this invention is to provide a spectral fundus images data measuring apparatus that reduces variation by the frequency in received light intensity, that keeps the received light intensity at the light receiving surface within a specified range, that is gentle on the subject eye, and that makes it possible to obtain favorable spectral characteristic.
Another object of this invention is to provide an apparatus and method for measuring spectral fundus images data that can eliminate position displacement between spectral images of the same part even if change in alignment occurs between the eye and the apparatus with the lapse of time.
Still another object of this invention is to provide an apparatus and method for measuring spectral fundus images data that can correct image positions almost fully automatically by a program using image correlation, and that can automate comparison between spectral images and display of spectral characteristics of characteristic points on the retina.